How can I increase the battery life on my HTC Desire.

Turn off the 3G when you aren't using it
Turn off wifi when not using it
Turn off bluetooth when not using it
download an app killer app such as ES task manager to stop apps running in the back ground that dont need to be running
run the screen brightness on medium or low

Hi, I had same problem which made phone unuseable for me, even after trying the usual things.. I bought a larger battery on ebay about £14 when I got mine a few months ago, more than doubles the capacity from 1400 mah to 3000 mah. Only drawback is case is thicker, however imho worth it to make it useable.

Seconding Juice defender. Also, try this: Get rid of all the widgets of your home screens, they are massive drains on battery. Install Advanced Task Killer pro and set it up to kill dormant apps every 30 mins.
If your phone is rooted, get Juice Defender pro and set it up to kill 3g data when screen is off, and to auto sync every hours rather than on demand. I did that on my Nexus 1 (Identical hardware) and was getting 2-3 days out of the battery with quite a lot of usage.

Depending on the apps you have, Task killers can make the battery situation worse instead of better in my experience, because if you're constantly using your phone, repeatedly starting up an app is going to be hell on the battery compared to just letting it sit in the RAM.

I also disagree with removing widgets because phones should be as functional and as customised as you want. It'd be like buying a fast car and never putting your foot down to save on petrol. My HTC Desire fast charges from 20% to 60% in an hour so if I forget to charge it in the evening, a quick burst as soon as I wake up before work is plenty to keep it going during the day.